Dynamic database memory management according to swap rates

ABSTRACT

Memory of a database management system (DBMS) that is running in a virtual or physical machine is managed using techniques that that reduce the effect of memory swaps on the performance of the physical or virtual machine. One such technique includes the steps of determining a swap rate while the database application is in an executing state, and decreasing the size of memory space available to the database application if the swap rate is above a threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 12/946,940, filed Nov. 16, 2010 and entitled “Method and System for Integrating Database Memory Management In Virtual Machines,” issued on Jan. 13, 2015 as U.S. Pat. No. 8,935,456 and U.S. patent application Ser. No. 12/946,971, filed Nov. 16, 2010 and entitled “Relieving Memory Pressure in a Host Using Database Memory Management” issued on Jan. 27, 2015 as U.S. Pat. No. 8,943,259.

BACKGROUND

Due to the specialized ways that database management systems (DBMS) utilize memory to access data, a DBMS typically implements its own memory management techniques rather than relying on more general memory management techniques that are provided by the underlying operating system on which the DBMS runs. For example, a DBMS may expressly request that the operating system allocate to it a portion of memory so that it can manage such memory on its own, thereby avoiding triggering of memory management techniques (disk swaps, LRU page replacement algorithms, etc.) that the underlying operating system may typically use to “over-commit” its available physical memory in an effort to provide running applications a larger “virtual” memory space in which to execute. That is, although the operating system may provide the DBMS a virtual memory space that is larger than the portion of physical memory allocated to the DBMS application, the DBMS application can, using its own memory management techniques, ensure that it utilizes its virtual memory space within the bounds of its allocated physical memory and therefore avoid any operating system level memory management activities that would otherwise adversely affect performance of the DBMS (e.g., untimely writes to swap disk, etc.).

Tuning the DBMS memory has been at the forefront of database research. Recently, autonomic techniques for tuning the memory of DBMS at runtime have been developed for some commercial relational databases. One such technique employs memory pools, each employing specialized paging policies apart from the paging policies of the operating system. Tuning parameters in this technique include the amount of memory to be allocated to these pools and how that memory is to be divided among the various memory pools.

The largest of the memory pools is the buffer pool, which contains the memory pages of database tables that are actively involved in transaction processing. As a transaction modifies rows in database tables, the pages containing these rows are brought into the buffer pool from disk and are modified in place. When the transaction is eventually committed by the DBMS, these “dirty” pages are flushed to disk under the control of the DBMS, for example, by atomically writing a record relating to committed transaction into a write-ahead transaction log on disk to ensure that the transaction's changes are never lost. It should be noted that the DBMS, not the operating system, determines when dirty pages of the buffer pool are written to disk.

In addition, the DBMS, implementing its own memory management, typically maintains its own free list of memory pages and memory page descriptor data structures that are separate and different from any memory management based free memory page lists and data structures maintained by the operating system. Indeed, a memory page that the DBMS may regard as free (e.g., because it has recently completed a database query transaction relating to the data in the memory page) may actually appear to the operating system to be a more important memory page because the DBMS has recently accessed the memory page. As an additional example, memory page descriptor data structures maintained by the DBMS may indicate which memory pages are “clean” and which ones are not. Clean memory pages are those that contain data that matches the corresponding data stored in the database on disk. Because the operating system has no knowledge that the DBMS utilizes portions of its allocated memory as an in-memory cache of the data it stores in the database on disk (e.g., for faster access and query response times), it is not able to similarly characterize the memory pages used by the DBMS.

The DBMS's own memory management techniques referenced above work well when the DBMS is the only application running on a host computer and is able to ensure its own allocation of physical memory. However, when a DBMS is run in a virtual machine that is hosted on a computer with other virtual machines, or the DBMS is run in a physical machine alongside other workloads, the DBMS, as initially tuned, may be forced to run with insufficient physical memory, resulting in performance degradations, e.g., due to disk swaps.

SUMMARY

One or more embodiments of the present invention provide techniques for managing memory of a database application running on an operating system in a virtual or physical machine that reduces the effect of disk swaps on the performance of the physical or virtual machine. A method according to an embodiment of the present invention includes the steps of determining a swap rate by the operating system while the database application is in an executing state, and decreasing the size of memory space available to the database application if the swap rate is above a threshold.

Further embodiments of the present invention include, without limitation, a non-transitory computer-readable storage medium that includes instructions that enable a processing unit to implement one or more aspects of the above methods as well as a computer system configured to implement one or more aspects of the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of a virtualized computer system according to an embodiment.

FIG. 2 is a conceptual diagram of the virtualized computer system of FIG. 1 that illustrates the method of managing memory of a database management system, according to an embodiment.

FIG. 3 is a flow diagram that illustrates the method of managing memory of a database management system, according to an embodiment.

FIG. 4 is a flow diagram that illustrates the method of resizing memory pools of a database management system running in a virtual machine, according to an embodiment.

FIG. 5 is a flow diagram that illustrates the method of adjusting a balloon memory target, according to an embodiment.

FIG. 6 is a performance graph illustrating the benefits of implementing the method of FIG. 5.

FIG. 7 is another performance graph illustrating the benefits of implementing the method of FIG. 5.

FIG. 8 depicts a block diagram of another computer system according to an embodiment.

DETAILED DESCRIPTION

FIG. 1 depicts a block diagram of a virtualized computer system according to an embodiment. Host computer system 100 may be constructed on a desktop, laptop or server grade hardware platform 102 such as an x86 architecture platform. Hardware platform 102 includes one or more central processing units (CPUs) 103, host physical memory 104 (also referred to as “machine memory”), and other standard hardware components (not shown). A hypervisor 114 is installed on top of hardware platform 102. Hypervisor 114 supports multiple virtual machine execution spaces 116 ₁-116 _(N), within each of which a VM process may be executed to instantiate corresponding VMs 120 ₁-120 _(N). For each of VMs 120 ₁-120 _(N), hypervisor 114 manages a corresponding virtual hardware platform (i.e., virtual hardware platforms 122 ₁-122 _(N)) that includes emulated hardware, such as virtual hard drive (HD) 124, one or more virtual CPUs 128, and virtual RAM 130 (also referred to as “guest physical memory”), that are illustrated in FIG. 1 for VM 120 ₁. Each virtual hardware platform 122 supports the installation of a guest operating system (OS) 132, which may be any commodity operating system that is compatible therewith. In each instance, the guest operating system provides user-level applications running in the virtual machine an interface to the virtual hardware platform of the virtual machine.

In the embodiments of the present invention described herein, a database management system (DBMS) 135 is running on guest OS 132 and includes various memory management modules. It should be recognized that one or more other applications (APPS) 113 may be running alongside DMBS 135 on guest OS 132 and compete for virtual hardware resources provided by virtual hardware platform 122 ₁. As will be further described below in conjunction with FIG. 2, the memory management modules of DMBS 135 communicate with a balloon driver 137 and a swap monitor 138 of guest OS 132 to carry out memory management. Balloon driver 137 is responsive to memory management commands from hypervisor 114, in particular a resource scheduler module 145 and operates as a balloon driver that is described in U.S. Pat. No. 7,433,951, entitled “System and Method for Controlling Resource Revocation in a Multi-Guest Computer System,” the entire contents of which are incorporated by reference herein. Swap monitor 138 is a kernel module inside guest OS 132 that, in one embodiment, computes the rate of guest physical memory pages that are swapped in from virtual HD 124. As will be described in further detail below, this swap rate is used as an indicator of when the guest (e.g., VM 120 ₁) is experiencing memory pressure. Moving weighted averages of several time intervals are used to filter out noise with more recent samples getting a higher weight. In this embodiment, swap out activity is ignored because “cold” pages (i.e., guest memory pages that have not been accessed recently) may be swapped out to virtual HD 124, and such swap activity may not be indicative of the guest coming under memory pressure. However, in some embodiments, swap monitor 138 may consider both the swap-in and swap-out activities in computing the swap rate.

It should be recognized that the various terms, layers and categorizations used to describe the virtualization components in FIG. 1 may be referred to differently without departing from their functionality or the spirit or scope of the invention. For example, virtual hardware platforms 122 ₁-122 _(N) may be considered to be part of virtual machine monitors (VMM) 140 ₁-140 _(N) which implement the virtual system support needed to coordinate operations between hypervisor 114 and their respective VMs. Alternatively, virtual hardware platforms 122 ₁-122 _(N) may also be considered to be separate (e.g., as a component of its corresponding virtual machine since such platforms include the hardware emulation components for the virtual machine) from VMMs 140 ₁-140 _(N), and VMMs 140 ₁-140 _(N) may be considered to be separate from hypervisor 114. One example of hypervisor 114 that may be used is included as a component of VMware's vSphere product, which is commercially available from VMware, Inc. of Palo Alto, Calif. (“VMware”). It should further be recognized that other virtualized computer systems are contemplated, such as hosted virtual machine systems, where the hypervisor is implemented in conjunction with a host operating system.

FIG. 2 is a conceptual diagram of the virtualized computer system of FIG. 1 that illustrates the method of managing memory of a database management system, according to an embodiment. The conceptual diagram of FIG. 2 depicts DBMS 135 running in VM 120 ₁ on top of guest OS 132, which includes balloon driver 137 and swap monitor 138. Resource scheduler 145 communicates with balloon driver 137 and, in the process of carrying out memory management for VMs 120, may instruct balloon driver 137 to “inflate” or “deflate.” The inflate command would be issued when host computer system 100 comes under memory pressure. The deflate command would be issued when the memory pressure has been relieved. Each inflate or deflate command includes an indication of a number of guest physical memory pages, referred to herein as a “balloon target.” The balloon target associated with an inflate or deflate command represents the number of guest physical memory pages (or corresponding amount of memory) that resource scheduler 145 would like to reclaim from guest OS 132 or return to guest OS 132, as the case may be.

DBMS 135 includes a balloon controller 232 which queries resource scheduler 145 on a periodic basis, e.g., once per second. In one embodiment, in order to perform such querying, balloon controller 232 leverages an application programming interface (API) 236 to access special runtime components (e.g., dynamically loaded libraries, shared memory objects, etc.) that have been installed in guest OS 132 that provide “backdoor” access to hypervisor 114 to obtain data about the state and performance of the virtual machine in which the application is running (e.g., such data that may be only known by hypervisor 114). One example of such an API is VMware's vSphere Guest API that interacts with the vSphere Guest SDK runtime components that are part of the VMware Tools product. It should be recognized that other techniques, such as utilizing hypercalls and other similar backdoor means for an application to communicate with hypervisor 114 may be utilized in other embodiments. Balloon controller 232 (via API 236) queries resource scheduler 145 for a variety of runtime information including the balloon target (and corresponding inflate or deflate request) and the host physical memory currently available (e.g., actually allocated) to VM 120 ₁. In another embodiment, balloon controller 232 may register itself with balloon driver 137 to receive the balloon target (and corresponding inflate or deflate request) directly from balloon driver 137 (and balloon driver 137 may accordingly be configured to notify or respond to balloon controller 232). It should be recognized that the host physical memory available to VM 120 ₁ may vary over time depending on the amount of host physical memory 104 used by all VMs 120 running in host computer system 100.

In one embodiment, in addition to assisting with inflate or deflate commands from resource scheduler 145, balloon controller 232, during normal operations of DBMS 135, continually manages and resizes the sizes of memory pools of DBMS 135 that have been initially tuned for optimal memory management with the VM 120 ₁, e.g., when DMBS 135 is the only application running in VM 120 ₁ and VM 120 ₁ is the only virtual machine running on hypervisor 114. The memory pools shown in FIG. 2 include buffer pool 208, sequential scan pool 212, and temporary table pool 216. In one embodiment, memory pools are executing components of DBMS 135 that have the responsibility of managing certain portions, of the memory allocated to DBMS 135 (e.g., each such portion, a “pool”), for example, by implementing policies that allocate or deallocate memory pages to the pool in a manner tailored to the particular usage of the pool by DBMS 135. It should be recognized, however, that memory pools may also refer to the allocated portion of memory itself (rather than the executing component), as the context requires. It should be also recognized that the three memory pools shown in FIG. 2 are for illustration and any number and type of memory pools may be provided in the embodiments of the present invention. In response to an inflate or deflate command originating from resource scheduler 145 or due to changes in available host memory to VM 120 ₁ during the normal course of execution of DBMS 135, balloon controller 232 may issue requests to any one (or all) of the memory pools to evict or allocate memory pages according to the memory pool's own memory management policies.

In one embodiment, each of the memory pools 208, 212, 216 maintains a page descriptor data structure. The page descriptor data structure distinguishes between free and used guest memory pages and marks used guest memory pages as dirty or not dirty (clean), i.e., identical to an on-disk image. In accordance with the memory pool's own memory management policies, the free memory pages may, for example, be preferentially used before evicting clean memory pages to avoid an extra disk read if the evicted clean page is accessed later, and clean memory pages may be preferentially evicted before dirty memory pages because the evictions would not require a corresponding write to a backing store, which would be a virtual hard drive 124 in this embodiment. When there are not enough free or clean memory pages, dirty memory pages will need to be selected for eviction, and these will require a corresponding write to the backing store.

It should be recognized that some memory pools of DBMS 135 (not shown in FIG. 2) do not maintain page descriptor data. One example is a private memory pool that is often used by sort operators. The private memory pool is not tracked on a per page basis. Instead, DBMS 135 maintains other statistics on the usefulness of private memory usage, e.g., how much sort memory is actively used, the rate of sort or hashing operators, etc.

The memory portion managed by buffer pool 208 operates as a large cache for accessing the actual data of the database that is stored on disk. For example, database tables and indexes may be cached by buffer pool 208 so that query transactions can be serviced by DBMS 135 in-memory without accessing the slow database disk. Buffer pool 208 may implement any policy for evicting and allocating memory pages. In one embodiment, memory pages from buffer pool 208 are evicted in the context of a process or thread of balloon controller 232. In an alternative embodiment, balloon controller 232 parallelizes this task by distributing its parts across multiple backend threads or processes.

DBMS 135 may also use a sequential scan pool 212, for example, to manage memory to facilitate operations that would perform large sequential scans of the data stored on the database disk. For example, a database query that cannot use a previously created database index might scan an entire database table on disk. Sequential scan pool 212 is used in such instances to prevent such operations from polluting buffer pool 208 with cached pages of data from the database that are not likely to be accessed in the near future. In a typical implementation, sequential scan pool 212 employs a special fixed-size ring data structure and an allocation or eviction request from balloon controller 232 causes a resizing of this ring data structure.

Temporary table pool 216 manages memory for temporary tables that have a lifetime that may be limited to either a session or a transaction, generally do not generate write-ahead log records, and cannot be recovered in the event of a crash. Temporary table pool 216 can thus be resized independently of buffer pool 208 in response an allocation or eviction request from balloon controller 232.

Balloon controller 232 also registers with a swap monitor 138 to obtain the rate of guest physical memory pages that are swapped in from virtual HD 124 (hereinafter referred to as the “guest swap rate”), which is computed by swap monitor 138 as a moving weighted average. The guest swap rate provides an indication of when the guest, e.g., VM 120 ₁, is under memory pressure. The guest may experience memory pressure for a variety of reasons. First, host computer system 100 may be under memory pressure and take memory away from the guest via the ballooning mechanism discussed above. Second, other workloads running in the guest, such as APPS 113, may increase, or the workload of DMBS 135 itself changes. Third, the guest may have been configured with too little memory for the workloads that it is servicing.

When the guest swap rate is below a threshold, memory of DBMS 135 is managed in the manner described below in conjunction with FIGS. 3 and 4. When the guest swap rate increases above a certain threshold, memory of DBMS 135 is further managed in the manner described below in conjunction with FIG. 5.

FIG. 3 is a flow diagram that illustrates a method of managing memory of a database management system that integrates DBMS memory management with virtual machine memory management. In the embodiment of the method described herein, the steps of the method are carried out by hypervisor 114, guest OS 132, DBMS 135, and specific components thereof. The method begins when hypervisor 114 decides to reallocate memory from VM 120 ₁ to another VM.

At step 310, resource scheduler 145 issues inflate commands to balloon drivers running in VMs 120, including balloon driver 137 of VM 120 ₁. The inflate command to balloon driver 137 includes a balloon target. At step 312, balloon controller 232 determines this balloon target (and the inflate command), for example, by querying resource scheduler 145 via its periodic polling through the aforementioned API (or receiving it directly from balloon driver 137). At step 314, balloon controller 232 issues an eviction request to each of the memory pools 208, 212, 216, in response to which the memory pools 208, 212, 216 each evict memory pages according to the policies implemented in them (step 315). It should be recognized that in alternative embodiments balloon controller 232 may only request certain memory pools to evict memory pages. Similarly, in alternative embodiments, balloon controller 232 may specifically request a number of memory pages (or amount of memory) for eviction depending upon the value of the balloon target. At step 316, balloon controller 232 releases the memory pages evicted by the memory pools 208, 212, 216 at step 314 to guest OS 132. In one embodiment where guest OS 132 is Linux, this is accomplished by madvise (MADV_REMOVE), which frees a range of guest memory pages and associated backing store. Afterwards, guest OS 132 is free to reuse these guest memory pages for any tasks including pinning such memory pages (e.g., so that they are not subsequently paged out to disk during performance of memory management by guest OS 132) and allocating them to balloon driver 137 in response to an inflate command from resource scheduler 145, which occurs at step 318. At step 320, after the memory pages evicted from the memory pools 208, 212, 216 have been pinned and allocated to balloon driver 137 (and balloon driver 137 has notified hypervisor 114 thereof), hypervisor 114 re-allocates these pinned memory pages to another VM according to conventional ballooning techniques. The number of memory pages freed by DMBS 135 while carrying out steps 312 through 316 may satisfy all or part of the balloon target. If it is less than the balloon target, the rest is satisfied by balloon driver 137 according to the standard ballooning techniques, such as those described in U.S. Pat. No. 7,433,951.

In one embodiment, balloon controller 232 implements a proportional allocation policy to preserve, over the course of the execution of DBMS 135, the relative memory pool sizes with respect to each other and to the total amount of host physical memory currently available (e.g., currently allocated) to VM 120 ₁ as this amount may change in response to other VM activity on host computer system 100. For example, as previously discussed, balloon controller 232 periodically queries resource scheduler 145 for the available host physical memory to VM 120 ₁ in order to implement this policy. This policy relies on results from an initial DBMS tuning performed with VM 120 ₁ running as the stand-alone virtual machine so that there is no competition for resources with other virtual machines running on top of hypervisor 114. Any technically feasible method for DBMS tuning on a host computer system may be carried out to obtain the initial memory pool sizes. From each of the initial memory pools sizes, a ratio reflecting the memory pool size to the total amount of host physical memory available to VM 120 ₁ in the absence other VMs competing for host physical memory. The computed ratios represent the “target” ratios for the memory pools while DBMS 135 is executing. During execution, balloon controller 232 periodically (e.g., as regular intervals) re-computes the actual ratios using the current pool memory sizes and the total amount of host physical memory available to VM 120 ₁ at the time of recomputation Any deviations from the target ratios trigger resizing of the affected memory pools via memory pool-specific allocation and eviction operations.

FIG. 4 is a flow diagram that illustrates the method of resizing memory pools of a database management system running in a virtual machine, according to an embodiment. In the embodiment of the method described herein, the steps of the method are carried out by balloon controller 232 of DBMS 135. The method is periodically triggered at regular intervals during the course of execution of DBMS 135.

At step 410, balloon controller 232 determines the host physical memory currently available to VM 120 ₁ by, for example, querying resource scheduler 145 as previously discussed. At step 412, balloon controller 232 computes the ratios for each of the memory pools 208, 212, 216 based on its current size and the host physical memory available to VM 120 ₁ determined in step 410. At step 416, balloon controller 232 determines if the computed ratio of any memory pool is greater than the target ratio for that memory pool. If yes, the method proceeds to step 424, where balloon controller 232 issues an eviction request to the memory pool whose computed ratio is greater than its target ratio, requesting that the memory pool evict an appropriate amount of memory pages so that the computed ratio matches (or more closely matches) the target ratio, and then to step 440. If no, step 420 is carried out. At step 420, balloon controller 232 determines if the computed runtime ratio of any memory pool is less than the target ratio for that memory pool. If yes, the method proceeds to step 422, where balloon controller 232 issues a allocation request to the memory pool whose computed runtime ratio is less than its target ratio, requesting that the memory pool allocate for itself more memory pages so that the computed ration matches or more closes matches the target ratio. The method then ends. If no, the method skips step 422 directly to the end. At step 440, balloon controller 232 releases to guest OS 132 the memory pages evicted by the memory pools 208, 212, 216 pursuant to any eviction requests. As described above, in one embodiment where guest OS 132 is Linux, this is accomplished by madvise (MADV_REMOVE), which frees a range of guest memory pages and associated backing store. It should be recognized that the method of FIG. 4 may further be used by balloon controller 232 in the context FIG. 3 (namely steps 314-316) to determine which memory pools (and how many memory pages in such memory pools) should be requested to proportionally evict memory pages to assist with inflate commands originating from resource scheduler 145 (or, conversely, proportionally re-allocate memory pages to memory pools to take advantage of a deflate command originating from resource scheduler 145). The method ends after step 440.

When the balloon target changes at a fast rate, maintaining the requested eviction rate may be challenging for balloon controller 232 if a substantial number of pages are dirty. For example, when memory pools are backed by conventional hard disk storage, the rate of eviction of dirty pages is limited by the storage I/O bandwidth. Rotating disks are especially unforgiving for random accesses and online transaction processing (OLTP) loads generally result in buffer pool populated in random order. Several possible optimizations to the techniques described above may be applied.

In one embodiment, if changes to the eviction order are tolerable, the eviction policy might be adjusted in favor of evicting clean memory pages over dirty ones. Since evicting clean memory pages does not require writes to the backing store, this increases the speed of memory page eviction.

In another embodiment, the policy of the preferential clean memory page eviction may be modified such that only a fraction of all clean memory pages are evicted preferentially with the rest of the balloon target satisfied from dirty memory pages to limit the damage to the DBMS working set.

In some embodiments, it may be possible for DBMS 135 to identify a sufficient number of eviction candidates before writing any dirty memory pages to the backing store. If it is possible, the dirty memory pages to be evicted are sorted by their position on disk (disk block numbers) and the writes are performed according to the sorted order and merged where possible.

In a further embodiment, to avoid a sharp drop in transaction throughput, balloon controller 232 may employ a technique of writing out dirty memory pages in bulk to temporary files in a sequential fashion while maintaining indexing data structures pointing to their temporary locations. These memory pages can then be lazily paged in from the temporary locations over a longer time period.

The method illustrated in FIG. 5 is carried out by balloon controller 232 to adjust the balloon size with respect to the balloon target received from resource scheduler 145 to account for situations where the guest swap rate increases above a certain threshold, which may be tuned according to the expected workload of DBMS 135. In one implementation, the threshold for the guest swap rate is 10 pages/second.

Balloon controller 232 periodically checks the guest swap rate computed by swap monitor 138, e.g., once per second (steps 511 and 512). If it determines at step 512 that the guest swap rate exceeds the threshold, balloon controller 232 enters a “swap inflate” state and increases the balloon size with respect to the balloon target (i.e., the current balloon target that it previously obtained via API 236 or from balloon driver 137) at step 513. In one embodiment, the rate of increase is made proportional to the guest swap rate. In one embodiment, the rate of increasing the size of memory space available to the database application is an exponentially increasing rate. After this increase, the flow returns to step 511 where balloon controller 232 waits another second before checking the guest swap rate again.

If, at step 512, balloon controller 232 determines that the guest swap rate does not exceed the threshold, it enters a “swap balance” state and proceeds to step 521. During the “swap balance” state, balloon controller 232 periodically checks the guest swap rate computed by swap monitor 138, e.g., once per second (steps 521 and 522). If it determines at step 522 that the guest swap rate exceeds the threshold, balloon controller 232 enters the “swap inflate” state once again and increases the balloon size above the balloon target at step 513. On the other hand, if it determines at step 522 that the guest swap rate does not exceed the threshold, step 523 is executed, where balloon controller 232 checks (e.g., via API 236) to see if the balloon target has been increased. If the balloon target has been increased, the flow returns to step 410 of FIG. 4. If the balloon target has not been increased, balloon controller 232 checks a timer to see if it has spent a minimum required amount of time in the “swap balance” state. In one embodiment, the minimum required amount of time is set in the timer as 10 seconds and the timer counts down. The “swap balance” state is implemented to reduce oscillations between the “swap inflate” state and a “swap deflate” which is described below. When the timer expires as determined at step 524, balloon controller 232 enters the “swap deflate” state. If the timer has not expired, the flow returns to step 521.

While in the “swap deflate” state, balloon controller 232 continuously decreases the balloon size at step 531 until it determines that one of the conditions judged at steps 532 and 533 is satisfied. At step 532, balloon controller 232 judges whether or not guest swap rate exceeds the threshold. If the guest swap rate exceeds the threshold, balloon controller 232 enters the “swap inflate” state once again and increases the balloon size at step 513. If the guest swap rate does not exceed the threshold, balloon controller 232 executes the decision block at step 533 where it checks (e.g., via API 236) to see if the balloon target has been increased or if the balloon size is equal to the balloon target (i.e., the balloon has been deflated to the current balloon target that it previously obtained via API 236 or from balloon driver 137 via step 531). If either of these conditions is satisfied, the flow returns to step 410 of FIG. 4. If not, balloon controller 232 continues to decrease the balloon size at step 531.

The technique described above in conjunction with FIG. 5 can be extended in several ways. First, the technique can be applied during normal operation in the absence of memory over-commit (e.g., balloon target is at zero). The occurrence of guest swap in such situations suggest that the guest may have been configured with less memory than required by the workloads running therein or that the DBMS load profile and/or query composition have changed over time. Second, the technique can be applied to a DBMS executing natively on hardware. This configuration is described below in conjunction with FIG. 8. Third, it should be recognized that the technique can be applied in the embedded database cases where the same VM is shared between DBMS and other workloads. Load spikes in the other workloads might trigger guest swap, even in the absence of host memory over-commit. Since the database is normally tuned in isolation without adjustments for load changes in other processes, external workload spikes might lead to guest swap. Fourth, the technique may be adapted to cause balloon controller 232 to eliminate guest swap but let the rest of the balloon target be drawn from guest kernel data structures and other user-level processes while still maintaining swap-free operation. Fifth, instead of waiting for guest swap-in activity, the technique may be guided by scanning rates of various guest kernel caches and use these scan rates as inputs in lieu of guest swap-in rates.

The benefits of the techniques described above in conjunction with FIG. 4 and FIG. 5 have been observed in a VM configured with 8 GB of RAM and a database buffer pool configured at 5.5 GB. In the experiment reflected in FIG. 6, the VM memory limit was lowered from 8 GB to 4 GB after 5 minutes (300 seconds) of execution. After a transitional period, the number of transactions executed in a VM that employs the inventive technique was observed to be about three times greater than the number of transactions executed in a VM that does not employ the inventive technique.

FIG. 7 shows the relative performance of a VM that employs the techniques described above in conjunction with FIG. 4 and FIG. 5 over a VM that does not employ such technique as the degree of over-commit increases. When memory over-commitment is at 0%, there is no difference in performance. When memory over-commitment is at 100%, the VM that employs the inventive technique outperforms the VM that does not employ the inventive technique by 2:1 margin. When memory over-commitment is at 700%, the margin becomes 5:1.

FIG. 8 depicts a block diagram of another computer system according to an embodiment. Computer system 800 may be constructed on a desktop, laptop or server grade hardware platform 802 such as an x86 architecture platform. Hardware platform 802 includes one or more central processing units (CPUs) 803, system memory 804, and other standard hardware components (not shown). An operating system (OS) 814 is installed on top of hardware platform 802, which may be any commodity operating system. A database management system (DBMS) 835 is running on OS 814 and includes the same memory management modules as DBMS 135. It should be recognized that one or more other applications (APPS) 813 may be running alongside DMBS 835 on guest OS 832 and compete for hardware resources provided by hardware platform 802. The memory management modules of DBMS 835 communicate with a swap monitor 838 of OS 814 to carry out the memory management technique described above in conjunction with FIG. 5.

In the embodiments described above, balloon controller 232 is implemented as a module within DBMS 135. It should be recognized that, in other embodiments, balloon controller 232 may be a standalone process dedicated to managing the memory pools of DBMS 135. In another embodiment, balloon controller 232 may reside in hypervisor 114, so that balloon controller 232 may be shared among two or more DBMS running in the same virtual machine or different virtual machines. Such sharing of balloon controller 232 permits the different instances of DBMS to reclaim memory on behalf of hypervisor 114 but in a DBMS-specific fashion.

Furthermore, in alternative embodiments of the present invention, guest memory freed by DBMS 135 may be released directly by a user-level thread or process running in DBMS 135, without calling into a kernel driver of guest OS 132. This can be achieved, for example, by extending hypervisor 114 with a hyper-call (a special existing instruction or any trapping instruction), which the user-level thread or process running in DBMS 135 can execute to trap into hypervisor 114 and release memory directly.

The various embodiments described herein may employ various computer-implemented operations involving data stored in computer systems. For example, these operations may require physical manipulation of physical quantities usually, though not necessarily, these quantities may take the form of electrical or magnetic signals where they, representations of them, are capable of being stored, transferred, combined, compared, or otherwise manipulated. Further, such manipulations are often referred to in terms, such as producing, identifying, determining, or comparing. Any operations described herein that form part of one or more embodiments of the invention may be useful machine operations. In addition, one or more embodiments of the invention also relate to a device or an apparatus for performing these operations. The apparatus may be specially constructed for specific required purposes, or it may be a general purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general purpose machines may be used with computer programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required operations.

The various embodiments described herein may be practiced with other computer system configurations including hand-held devices, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, and the like.

One or more embodiments of the present invention may be implemented as one or more computer programs or as one or more computer program modules embodied in one or more computer readable media. The term computer readable medium refers to any data storage device that can store data which can thereafter be input to a computer system computer readable media may be based on any existing or subsequently developed technology for embodying computer programs in a manner that enables them to be read by a computer. Examples of a computer readable medium include a hard drive, network attached storage (NAS), read-only memory, random-access memory (e.g., a flash memory device), a CD (Compact Discs) CD-ROM, a CD-R, or a CD-RW, a DVD (Digital Versatile Disc), a magnetic tape, and other optical and non-optical data storage devices. The computer readable medium can also be distributed over a network coupled computer system so that the computer readable code is stored and executed in a distributed fashion.

Although one or more embodiments of the present invention have been described in some detail for clarity of understanding, it will be apparent that certain changes and modifications may be made within the scope of the claims. Accordingly, the described embodiments are to be considered as illustrative and not restrictive, and the scope of the claims is not to be limited to details given herein, but may be modified within the scope and equivalents of the claims. In the claims, elements and/or steps do not imply any particular order of operation, unless explicitly stated in the claims.

Plural instances may be provided for components, operations or structures described herein as a single instance. Finally, boundaries between various components, operations and data stores are somewhat arbitrary, and particular operations are illustrated in the context of specific illustrative configurations. Other allocations of functionality are envisioned and may fall within the scope of the invention(s). In general, structures and functionality presented as separate components in exemplary configurations may be implemented as a combined structure or component. Similarly, structures and functionality presented as a single component may be implemented as separate components. These and other variations, modifications, additions, and improvements may fall within the scope of the appended claims(s). 

We claim:
 1. In a computer system having an operating system and a database application running as a user-level application on top of the operating system, a method of dynamically adjusting a size of memory space available to the database application, said method comprising the steps of: determining a swap rate of memory pages swapped into memory from disk by the operating system while the database application is in an executing state; and decreasing the size of memory space available to the database application if the swap rate is above a threshold, wherein the decreasing is carried out at a rate proportional to the swap rate.
 2. The method of claim 1, wherein said decreasing is carried out at the same rate as the swap rate.
 3. The method of claim 1, wherein the swap rate is a weighted average of multiple swap rates obtained over a period of time.
 4. The method of claim 1, further comprising: if the swap rate is below the threshold for a period of time, increasing the size of memory space available to the database application.
 5. The method of claim 4, wherein the increasing is done at an exponentially increasing rate.
 6. The method of claim 1, wherein the computer system is a virtual machine running in a physical machine.
 7. The method of claim 1, wherein the computer system is a physical machine.
 8. A non-transitory computer readable storage medium comprising instructions for causing a computer system having an operating system and a database application running as a user-level application on top of the operating system to carry out a method of dynamically adjusting a size of memory space available to the database application as a function of swap rate, said method comprising the steps of: determining a swap rate of memory pages that are swapped into memory from disk by the operating system while the database application is in an executing state; and decreasing the size of memory space available to the database application if the swap rate is above a threshold, wherein the decreasing is carried out at a rate proportional to the swap rate.
 9. The non-transitory computer readable storage medium of claim 8, wherein said decreasing is carried out at the same rate as the swap rate.
 10. The non-transitory computer readable storage medium of claim 8, wherein the swap rate is a weighted average of multiple swap rates obtained over a period of time.
 11. The non-transitory computer readable storage medium of claim 8, wherein the method further comprises: if the swap rate is below the threshold for a period of time, increasing the size of memory space available to the database application.
 12. The non-transitory computer readable storage medium of claim 11, wherein the increasing is done at an exponentially increasing rate.
 13. A method of managing memory of a database management system running in a virtual machine instantiated on a host computer having machine memory, wherein the database management system partitions an allocated memory address space into a plurality of memory pools and an initial size for each of the memory pools has been determined based on an initial allocation of guest physical memory to the virtual machine by the host system and a purpose for which the memory pool has been created, the method comprising: periodically determining a current amount of guest physical memory allocated by the host computer to the virtual machine during execution of the database management system; adjusting downward the current amount of guest physical memory allocated by the host computer to the virtual machine at a rate proportional to a swap rate of memory pages that are swapped into guest physical memory from disk in the virtual machine in response to the swap rate being above a threshold; determining, for each memory pool, a current memory allocation measurement based on a current size of the memory pool and the determined current amount of guest physical memory; requesting a release of a portion of memory from a memory pool if the determined current memory allocation measurement for the memory pool is greater than a target memory allocation measurement for the memory pool, wherein the target memory allocation measurement is based on the initial size of the memory pool and the initial allocation of guest physical memory to the virtual machine; and requesting an additional allocation of a portion of memory for a memory pool if the determined current memory allocation measurement for the memory pool is less than the target memory allocation measurement for the memory pool.
 14. The method of claim 13, wherein the swap rate is a weighted average of multiple swap rates obtained over a period of time.
 15. The method of claim 13, wherein the current amount of guest physical memory allocated by the host computer to the virtual machine is adjusted downward at the same rate as the swap rate if the swap rate is above a threshold.
 16. The method of claim 13, further comprising: adjusting upward the current amount of guest physical memory allocated by the host computer to the virtual machine in response to the swap rate continuing to be below a threshold for a predetermined duration of time.
 17. The method of claim 16, wherein the current amount of guest physical memory allocated by the host computer to the virtual machine is adjusted upward at an exponentially increasing rate. 